For sorption of gaseous pollutants, two different methods are usually applied, the so-called dry method and the so-called wet-dry method.
As a rule, the dry method is applied in the combustion of waste and is distinguished in that a sorbent in the form of a dry, finely divided powder is supplied to the flue gases in a contact reactor. The sorbent can be, for instance, an absorbent, such as lime, which thus absorbs the acidifying components that are present in the flue gases, whereby particulate pollutants are formed, which together with the remaining so-called residual products are separated in a subsequent dust separator, such as a bag filter. To obtain an operating temperature that is suitable for the absorption, the flue gases are usually cooled by injection of water which is evaporated therein in a cooling tower arranged before the contact reactor. The wet-dry method is usually applied in the combustion of waste or coal and is distinguished in that a sorbent solution or sorbent suspension is injected into the flue gases in a contact reactor. Also in this case, the separation of the particulate pollutants formed takes place in e.g. a bag filter. An operating temperature that is suitable for the sorption is obtained by the water in the solution or suspension being evaporated in the flue gases. Both methods result in a completely dry residual product.
It is known that the sorption capacity and, thus, the degree of separation depends to a great extent on the temperature and moisture content at which the sorption takes place. The ratio of the temperature of the flue gases to the moisture content thereof can be expressed in their relative humidity or so-called condition.
It may be mentioned that relative humidity is, by definition, the ratio of the current partial pressure of the water vapour to the corresponding saturation pressure at the same temperature. The moisture content of the flue gases derives on the one hand from the inlet moisture content after combustion and, on the other hand, from the amount of water supplied for cooling the flue gases. With an increased moisture content, the sorption capacity increases, i.e. the tendency of the sorbent to react with the gaseous pollutants of the flue gases, which, for a satisfactory sorbent utilisation, makes it necessary to decrease the temperature as much as possible. There is, however, an upper limit for the moisture content in respect of the separated residual products. At too high a moisture content, the residual products will be moist and, thus, sticky, which results in clogging of the filter bags of the bag filter. The residual products will also be practically impossible to handle. The hygroscopic calcium chloride formed by the reaction between lime as absorbent and the hydrogen chloride of the flue gases is very important to the moisture content of the separated residual products. On the other hand, too low a moisture content reduces the sorption capacity, which makes the sorbent consumption increase for a given degree of separation. Thus, it is difficult to control the condition of the flue gases to a level which is optimum for the process concerned.
The temperature and moisture content of the flue gases as well as the amount of acidifying components often fluctuate widely, among other things owing to great variations in the composition of the fuel that is being burnt, especially in the combustion of waste. These fluctuations may also be very rapid and thus result in emission peaks, i.e. temporarily high outlet contents of the gaseous pollutants of the flue gases. The controlling of the condition of the flue gases to a level that is optimum for the process concerned thus is rendered still more difficult.
According to conventional technique in, for instance, the dry method, the temperature in the dust separator is set at a constant level by the required amount of water being controlled and injected into the flue gases, the temperature of the flue gases being continuously measured, preferably directly after the cooling tower. Since the moisture content of the flue gases fluctuates as described above, the condition thereof cannot be controlled by this technique. To avoid the sticky residual products. the average value of the moisture content must therefore be selected by a great margin in relation to the defined saturation curve of the calcium chloride when lime is used as absorbent, which thus implies that a relatively high constant temperature must be selected. Consequently, this results in a lower degree of separation and/or a higher consumption of sorbent. The lime is injected into the flue gases as usual in a contact reactor arranged between the cooling tower and the dust separator. The amount of lime is supplied with respect to the content of acidifying components. Since a control of the emission of acidifying components is only connected to the supply of lime and not integrated with the control of the water supply or the cleansing, it is thus not possible to achieve optimum conditions for the separating of acidifying components. Owing to inertia in the control system and the lime in itself, this has a slow response in respect of the absorption of acidifying components, especially in case of emission peaks. It thus takes a relatively long time before a sufficient amount of lime has been supplied to the flue gases to overcome the content of acidifying components, which increases in case of an emission peak. That stated above will appear from the curves A, B and C0 in FIG. 1, the sum of the times t1 and t2 being the total response time of the lime.
It is thus also difficult to satisfy the greater and greater requirements that are placed on emission guarantees with permissible maximum average values over short times, such as the new EU requirements which are connected to 24-hour and 30-minute average values. The 24-hour average values for hydrogen chloride and sulphur dioxide according to these new rules will be, for instance, 10 and 50 mg/Nm.sup.3, respectively, and the corresponding 30-minute average values will be 60 and 200 mg/Nm.sup.3, respectively. It is also difficult to comply with customers' requirements, which do not allow emission to exceed a given maximum momentaneous limit value.
EP 596 229 discloses a method concerning the dry and the wet-dry method for cleaning flue gases formed in the combustion of waste. Here, use is made of a calcium-containing absorbent for separating the acidifying components of the flue gases. To achieve a favourable condition and at the same time prevent the separated residual products from being overmoistened owing to hygroscopic calcium chloride, the inlet temperature and moisture content of the flue gases to the contact reactor as well as inlet gas flow are measured. A quantity of water that is necessary for this purpose can be determined in consideration of the corresponding measured values of the outlet temperature and the possible moisture content of the flue gases leaving the contact reactor. The quantity of water is controlled in this manner and injected into the flue gases for cooling to a predetermined operating temperature and moisture content, i.e. condition, which thus lies at a predetermined constant basic level relative to the saturation curve defined for the calcium chloride. As a result, the problem with the forming of overmoistened, sticky residual products is in fact eliminated. However, the relatively great distance between the above-mentioned constant level and the saturation curve of the calcium chloride does not result in an optimum absorption capacity, which leads to a lower degree of separation and, consequently, a higher consumption of absorbent in order not to exceed the set limit values of the emission, especially when the emission temporarily has high contents of acidifying components. The control system for the condition of the flue gases will also be relatively complicated and expensive to accomplish since it requires extensive measuring equipment for measuring the above-mentioned parameters. Besides, the consumption of absorbent will be higher since the monitoring of the emission and the controlling of the cleansing are not integrated with this control system. A higher consumption of absorbent implies an increased amount of unused absorbent in the residual products, which results in increased costs for the provision of absorbent as well as the deposition of these residual products.